Hi guys!
First of all, you have a really awesome hobby! I am planning to buy a simple coaxial RC-heli and join.
Recently I saw a bunch of videos on Youtube with amazing stunts performed by RC helicopters pilots, and my interest grew rapidly. That's why I showed one of them to a friend.
To my big surprise he said that it must be fake and was absolutely sure about that. Then I've shown him many other videos with similar stunts (I think it is called 3d aerobatics?), but this didn't convince him at all. He is sure that this type of flying is physically impossible.
As for me, I am sure it is all real, since there are thousands of videos (also in HD), websites, and forums (like this one) on the topic. It would cost billions of dollars to create all this 'illusion', and there is no sense to do it.
He is especially skeptic about one stunt - when the helicopter hovers with its rotor being perpendicular to the ground. I attached a screenshot to the thread.
He can't understand which forces hold heli still in this moment.
Initial googling helped me to find out that this can be done only with collective pitch models.
I understand that changing pitch you can stabilize the heli, but where does the thrust, which prevents it from falling down comes from?
I need your help to get physics behind this stunt and finally convince my friend.

First of all, it wouldn't take "billions" of dollars to make a video of a fake heli performing 3D stunts......today's digital graphic software provides plenty of "realism" in many big screen productions for under a couple of K's........
Second of all, it's obvious a "non-believer" has not been around the RC heli sport, or for that matter, around any live RC aerial event. So don't knock yourself out trying to convnce one.......

Third of all, only a small percentage of RC heli pilots are able to perform the type of "tricks" shown on the posted video. It takes years of practice, very good and expensive equipment.....not to mention a "God" given physical/mental ability to perform consistantly and over long periods of time.......for every 5 guys who are able to do this stuff, there are 1000 that will never be able to.

So, it's good your'e excited about the hobby, and starting conservative is a very wise idea......by the way, the "thrust" is provided by the heli's main blades pitch and speed.

Thank you for advice, guys!
The problem is, 'a nearby club' is about 800 km away. Otherwise I would definitely bring him there.
That's why I want to explain him (and myself) this stunt with some theory. I have just drawn a picture to better explain the problem.
What I am looking for is an explanation of this mystery force against gravity.

P.S. Just noticed that I posted the thread to the wrong section (airplanes)

Yeah, when I first saw 3D heli flying in the flesh I struggled to believe my eyes. I honestly thought the heli was out of control and in the process of crashing. Only when the mayhem went on for minutes without an actual crash occuring did I accept that the guy on the sticks actually had control!

The heli is only in that spot for a fraction of a second. It is called a tick-tock (IIRC) and it has just enough force as it goes back and forth to maintain altitude. They are pretty fun to watch and do appear to defy gravity for a while.

What I am looking for is an explanation of this mystery force against gravity.

The simple answer is that in that instant in time captured in the photo there is no lift from the rotor acting against gravity. If the heli stayed stationary in that attitude it would fall to the ground. The thing to bear in mind though is that the heli doesnt stay in that position for more than a brief moment, it's most likely pivoting violently from side to side in a 'tick-tock'. When at each end of it's oscillation the rotors can make lift because they are no longer vertical.

Think of it like a tennis ball being volleyed back and forth over the net. While it's in flight it has no force to 'lift' other than it's own momentum, but at each end of its oscillation is gets a whack with a racket that throws it up in the air again meaning that it never falls to the ground.

The power needed to perform those maneuvers is very high and there is a huge amount of energy stored in the high speed rotating blades .

Also similar to whacking a baseball with a bat. You can not throw a baseball 350 feet even if you are on steroids. The bat stores up energy imparted by the player over time and instantly transfers that energy to the baseball for a home run shot into the outfield bleachers.

By the way, the cheapie co-axial heli's you see in the department stores will not perform the stunts you see these 3D heli's doing. Do not even attempt. These heli's you're viewing all have collective pitch. Can look at it as if it were a wing with ailerons and an elevator was rotating.

First of all, Get rid of your non believer friend LOL, Ok Just Kidding
Your friend is Entitled to his or her own opinion, Dont even try to convince them, it will take all the fun out of this wonderful hobby, get good at flying rc helicopters, and show them what they can do, get a good simulator to practice flying, it will save you a ton of money learning to fly, take care, Chellie

I may be getting Older, But I Refuse to grow Up I am Having to much Fun to Grow Up LOL

There are all kinds of people with all kinds of mind sets. A lot of people are simply not interested in aviation in general and some seem to have a revulsion for anything technical or mechanical. There are even some who refuse to learn to drive a car or even ride a bicycle. Have you ever tried to discuss model planes or helis with sports- minded people, many seem uninterested in anything else. Do you have any interest in collecting bottle caps or stamps? I sure don't!